1. Technical Field
The present invention generally relates to the conversion of light gases such as carbon dioxide and/or methane into hydrocarbons and/or liquid oxygenates. The invention relates more particularly to apparatus and methods for producing liquid oxygenates and/or hydrocarbons from a light gas such as carbon dioxide and/or methane by high shear contacting of reactants.
2. Background of the Invention
The effect of increasing carbon dioxide emission on global warming is a major concern of scientists and governments due to its effect on the environment. The increased use of fossil fuels as a source of power and heat is the main reason for the increase in carbon dioxide emissions. The combustion of fossil fuels is an exothermic process where the energy released is typically used for heating and/or conversion to other forms of energy such as mechanical energy. Oxidation of hydrocarbons is also common practice in chemical reactions such as oxidation of ethylene, Fischer Tropsch and other reactions. The resulting effluent from combustion of hydrocarbon depends on the make up of the hydrocarbon but is mainly carbon dioxide and water. Releasing large amounts of carbon dioxide into the atmosphere is believed to be responsible for adverse effects to the environment and there are efforts underway to reduce carbon dioxide emissions to help abate these negative effects.
A viable solution to the deleterious environmental effects of carbon dioxide emissions should result in a net reduction of carbon dioxide emissions. Technologies to sequester carbon dioxide can consume large amounts of energy, the energy, in many cases, derived from fossil fuels, and thus resulting in little or no net reduction in carbon dioxide, or worse yet a net increase in carbon dioxide production.
A process that allows recycling carbon dioxide to produce a valuable product such as fuel or chemical feedstock would be of great benefit in reducing the purported effects of carbon dioxide on global warming. It would be additionally beneficial to develop a process to convert carbon dioxide into a liquid fuel that can be transported and/or used as a feedstock for refinery or petrochemical processes.
Methane is an important building block in organic reactions used in industry as well as an important fuel source. The methane content of natural gas may vary within the range of from about 40 volume percent to about 95 volume percent. Other constituents of natural gas may include ethane, propane, butanes, pentane (and heavier hydrocarbons), hydrogen sulfide, carbon dioxide, helium and nitrogen. Natural gas in liquid form has a density of 0.415 and a boiling point of minus 162° C. It is therefore not readily adaptable to transport as a liquid except for marine transport in very large tanks with a low surface to volume ratio. Large-scale use of natural gas often requires a sophisticated and extensive pipeline system. A significant portion of the known natural gas reserves is associated with remote fields, to which access is difficult. For many of these remote fields, pipelining to bring the gas to potential users is not economically feasible. Economically transporting methane from remote areas by converting the gas to a liquid has long been sought in the industry.
Indirectly converting methane to methanol by steam-reforming to produce synthesis gas as a first step, followed by catalytic synthesis of methanol is a well-known process. Aside from the technical complexity and the high cost of this two-step, indirect synthesis, the methanol product has a limited market and the process thus does not appear to offer a practical way to utilize natural gas from remote fields.
A process that provides an effective means for catalytically converting methanol to gasoline is described in U.S. Pat. No. 3,894,107 (Butter et al.). Although the market for gasoline is large relative to the market for methanol, and although this process is currently used in New Zealand, it is complex and its viability appears to be limited to situations in which the cost for supplying an alternative source of gasoline is high.
Attempts to carry out the partial oxidation of methane to liquid compounds (such as methanol or ethanol) in the gas phase have met with limited success because of difficulties in controlling the free radical processes that are involved. Since methanol is more reactive than methane, the undesirable formation of CO and CO2 via secondary combustion has been unavoidable. While a variety of catalysts, mostly metal oxides, have been reported for the partial oxidation of methane to methanol, the reaction has required high temperatures and the reported methanol yields based on methane have generally been less than 10%.
Indirect approaches for the conversion of methane to methanol have been reported by Bjerrum, U.S. Pat. No. 6,380,444; Periana, U.S. Pat. No. 5,233,113; and Chang, U.S. Pat. No. 4,543,434. The general reaction system used for these approaches utilize a small quantity of a radical initiator (acid) that will strip a hydrogen atom from methane, to generate methyl radicals and a small quantity of acid. Some patents have demonstrated that methane can be converted to methyl bisulfate in a single-step using Group VIII noble metal catalyst (such as platinum or palladium), and a strong inorganic acid such as sulfuric acid. Other patents describe processes which do not utilize catalyst in the conversion of methane to methanol (e.g., European Patent No. 1,558,353). Chlorine and other halogen containing acids have also been utilized in a similar manner to convert methane to methanol and other liquids. These processes tend to encounter problems with corrosion at elevated temperatures, produce relatively low yields of methanol, and create unwanted byproduct.
U.S. Pat. No. 7,282,603 to Richards discloses anhydrous processing of methane into methane sulfonic acid, methanol and other compounds and provides an overview of some of the past approaches to converting methane into methanol. The approach of Richards avoids the use or creation of water, and utilizes a radical initiator compound such as halogen gas or Marshall's acid to create methyl radicals.
Existing processes and production facilities for producing liquids from methane are typically subject to various constraints such as mass flow and product yield limitations and plant size and energy consumption requirements.
Accordingly, in view of the art, there is a need for efficient and economical methods and systems for converting carbon dioxide and/or low molecular weight alkanes, in particular methane, into valuable products whereby the emission of carbon dioxide into the environment may be reduced and/or a system and process whereby a light gas stream comprising carbon dioxide and/or methane may be converted into a liquid product. The greenhouse gas problem is addressed by the herein disclosed system and process for the conversion of carbon dioxide to hydrocarbons and/or oxygenates through the use of a high shear reactor. Such systems and methods should permit increased selectivity and yield of liquid oxygenates and conversion of methane and/or carbon dioxide, while allowing economically favorable conditions of operating temperature, pressure and/or reaction time.